1. Field of the Invention
This invention relates to heat exchangers, and more particularly to heat exchangers for cooling engines, generators, gear boxes and other heat generating sources in industrial apparatuses having fluid cooled heat sources, such as marine vessels. The invention more particularly relates to open heat exchangers (where heat transfer tubes are exposed to the ambient cooling or heating fluid, rather than being in a shell to shell container holding the cooling or heating fluid) used for cooling heat sources, where the heat exchangers are efficient, and thus have lower weight and volume compared to other heat exchangers known in the art. Alternatively, the heat exchanger according to the invention could be used as a heater, wherein relatively cool fluid absorbs heat through the heat transfer tubes.
2. Description of the Prior Art
Heat generating sources in industrial applications such as marine vessels are often cooled by water, other fluids or water mixed with other fluids. For example, in marine vessels used in fresh water and/or salt water, the cooling fluid or coolant flows through the engine or other heat generating source where the coolant picks up heat, and then flows to another part of the plumbing circuit. The heat must be transferred from the coolant to the ambient surroundings, such as the body of water in which the vessel is located. For small engines, such as outboard motors for small boats, ambient water pumped through the engine is a sufficient coolant. However, as the vessel power demand gets larger, ambient water pumped through the engine may continue to provide good cooling of the engine, but also serves as a source of significant contamination damage to the engine. If raw, ambient water were used to cool the engine, the ambient water would carry debris and, particularly if it is salt water, corrosive chemicals to the engine. Therefore, there have been developed various apparatuses for cooling engines and other heat sources. One apparatus for cooling the engine of a vessel is channel steel, which is basically a large quantity of shaped steel which is welded to the bottom of the hull of a vessel for conveying engine coolant and transferring heat from the coolant to the ambient water. Channel steel has severe limitations: it is very inefficient, requiring a large amount of steel in order to obtain the required cooling effect; it is very expensive to attach to a vessel, since it must be welded to the hull—a very labor intensive operation; since channel steel is very heavy, the engine must be large enough to carry the channel steel, rendering both the initial equipment costs and the operating costs very high; the larger, more powerful engines of today are required to carry added channel steel for their cooling capacity with only a relatively small amount of room on the hull to carry it; the payload capacity is decreased; the large amount of channel steel is expensive; and finally, channel steel is inadequate for the present and future demands for cooling modern day, marine vessels. Even though channel steel is the most widely used heat exchanger for vessels, segments of the marine industry are abandoning channel steel and using smaller keel coolers for new construction to overcome the limitations cited earlier.
A keel cooler was developed in the 1940's and is described in U.S. Pat. No. 2,382,218 (Fernstrum). The Fernstrum patent describes a heat exchanger for attachment to a marine hull structure which is composed of a pair of spaced headers secured to the hull, and a plurality of heat conduction tubes, each of whose cross-section is rectangular, which extend between the headers. Cylindrical plumbing through the hull connects the headers to coolant flow lines extending from the engine or other heat source. Hot coolant leaves the engine, and runs into a heat exchanger header located beneath the water level (the water level refers to the water level preferably below the aerated water, i.e. below the level where foam and bubbles occur), either beneath the hull or on at least one of the lower sides of the hull. The coolant then flows through the rectangular heat conduction tubes and goes to the opposite header, from which the cooled coolant returns to the engine. The headers and the heat conduction tubes are disposed in the ambient water, and heat transferred from the coolant, travels through the walls of the heat conduction tubes and the headers, and into the ambient water. The rectangular tubes connecting the two headers are spaced fairly close to each other, to create a large heat flow surface area, while maintaining a relatively compact size and shape. Frequently, these keel coolers are disposed in recesses on the bottom of the hull of a vessel, and sometimes are mounted on the side of the vessel, but in all cases below the water line.
The foregoing keel cooler is referred to as a one-piece keel cooler, since it is an integral unit with its major components welded or brazed in place. The one-piece keel cooler is generally installed and removed in its entirety.
It is explained in U.S. Pat. No. 2,382,218 that, according to one embodiment of the heat exchanger disclosed therein, the pair of headers at opposite ends thereof have beveled fore and aft front and rear end walls. The latter walls are respectively connected to beveled front and rear inner walls (to which the open ports of the conduction tubes are connected to the chambers of the headers) by triangular-shaped side walls. There is thus no flat lower wall (since the triangular-shaped end walls meet at a point). Each header thus essentially consists of a flat rectangular upper wall, beveled inner and end walls extending downwardly from opposite ends of the upper wall and meeting at a lower point, and triangular side walls. The coolant inlet or outlet nipple is positioned in the upper wall directly over the point where the beveled inner wall meets the beveled end wall. Since the walls beneath the nipples are beveled in opposite directions, the flow of coolant to or from the nipples is helped in one respect because the fore and aft end parts of the header assists the coolant flow between the conduction tubes and the nipple, but hindered in another respect because the beveled inner walls direct coolant flow in the opposite direction from that rebounding from the end parts of the header. The oppositely directed coolant flow in the header causes turbulence, increases pressure drop and reduces coolant flow. The lower portion of each header, where the beveled inner and end wall converge at a point, is disposed below the open ports of the conduction tubes. This further reduces the coolant flow into and out of the respective headers; in the case of coolant flowing out of a header, coolant disposed below the ports must flow upward to reach the ports, against other coolant flowing downwardly. Such upward flow contributes to turbulence in the header, and is inefficient since gravity opposes such flow. Likewise, when coolant is flowing into this type of header to exit through the nipple, some coolant flows in the direction opposite to that of the nipple and must flow against gravity, past the cross-flow of coolant from the ports of the conduction tubes into the nipple. The latter arrangement also results in turbulence in the header, an increase in the pressure drop, and reduces coolant flow. In addition, none of the open ports are located in the coolant flow path from the nipple, so there is no direct flow path between the nipple and any of the open ports.
Furthermore, the beveled end wall is not configured either to direct a substantial amount of coolant into the flow tubes, or to direct a substantial amount of coolant from the flow tubes into the nozzle. This is because part of the beveled end wall is located below the open ports to the flow tubes, and because there are significant parts of the beveled inner wall which are flat and devoid of ports to the flow tubes.
These aspects of this embodiment of the heat exchanger disclosed in U.S. Pat. No. 2,382,218 may very well have determined why it was never put into commercial production.
There are various varieties of one-piece keel coolers. Sometimes the keel cooler is a multiple-pass keel cooler where the headers and heat conduction tubes are arranged to allow at least one 180° change in the direction of flow, and the inlet and outlet ports may be located in the same header.
Even though the foregoing heat exchangers with the rectangular heat conduction tubes have enjoyed widespread use since their introduction over fifty years ago, they have shortcomings which are corrected by the present invention.
The rectangular heat exchangers of the prior art have the outward shape of a rectangular parallelepiped having headers at their opposite ends. These headers have opposing end walls which are perpendicular to the hull of the vessel and parallel to each other, and act as a barrier to ambient water flow relative to the keel cooler as the vessel with the heat exchanger travels through the water. The perpendicular header walls are responsible for the creation of dead spots (lack of ambient water flow) on the heat exchanger surfaces, which largely reduce the amount of heat transfer occurring at the dead spots. In addition, the perpendicular walls diminish the flow of ambient water between the heat conduction tubes, which reduces or diminishes the amount of heat which can be transferred between the coolant in the tubes and the ambient water.
As discussed below, the beveled header contributes to the increase of the overall heat transfer efficiency of the keel cooler according to the invention, since the ambient water is caused to flow towards and between the respective heat conduction tubes, rendering the heat transfer substantially higher than in the keel cooler presently being used. This increase in heat transfer is due at least in part to the increase in turbulence in the flow of ambient water across the forward header and along and between the coolant flow tubes.
One of the important aspects of keel coolers for vessels is the requirement that they take up as small an area on the vessel as possible, while fulfilling or exceeding their heat exchange requirement with minimized pressure drops in coolant flow. The area on the vessel hull which is used to accommodate a keel cooler is referred to in the art as the footprint. In general, keel coolers with the smallest footprint and least internal pressure drops are desirable. One of the reasons that the keel cooler described above with the rectangular heat conduction tubes has become so popular is because of the small footprint it requires when compared with other keel coolers. However, keel coolers according to the design of rectangular tubed keel coolers presently being used have been found by the present inventors to be larger than necessary both in terms of size and the related internal pressure drop. By the incorporation of the various aspects of the present invention described above (and in further detail below), keel coolers having smaller footprints and lower internal pressure drops are possible. These are major advantages of the present invention.
When multiple pass (usually two-pass) keel coolers are specified for the present state of the art, an even greater differential size is required when compared with the present invention, as described below.